1. Field of the Invention
The present invention relates to an apparatus and method for pulverizing a dry toner for developing a latent electrostatic image in an electrophotography, electrostatic recording and electrostatic printing, and particularly relates to a process for making a toner raw material into a fibrous shape with a method in which the toner raw material is made into the fibrous shape and pulverized to obtain particles.
Moreover, the present invention relates to a method for producing fine resin particles to produce fine particles having a uniform particle size distribution, and fine resin particles obtained by the method, and particularly, relates to a method for pulverizing a dry toner for developing a latent electrostatic image in electrophotography, electrostatic recording and electrostatic printing.
2. Description of the Related Art
Recent years, in electronic printing and electrophotographic fields, marketing needs for high resolution have been increasing. As a toner used for printing, fine resin particles having a narrow particle size distribution are needed to be used in order to improve resolution of images and letters printed on a paper sheet by using electronic devices such as copiers and printers.
A technique in which a resin used for the toner is made into uniform fine particles is essential to obtain the fine resin particles.
Conventionally, an apparatus for producing fine resin particles used for toners mainly includes: (1) an adding and kneading unit configured to add a colorant, pigment, charge controlling agent, releasing agent, hardening agent and other additives to a resin and knead them; (2) a pulverizing unit configured to pulverize the kneaded resin; and (3) a classifying unit configured to classify the pulverized resin.
However, by the apparatus including kneading, pulverizing, and classifying units, it has been difficult to obtain fine resin particles having narrow particle size distribution which meet the marketing needs. Actually, conventional fine resin particles used for toner have an average particle diameter of 5 μm to 8 μm, and it has been difficult to obtain fine resin particles having a narrow particle size distribution in good yield by the above apparatus. This is because the resin is excessively pulverized during pulverization, and additionally, because a large amount of resin particles that fall out of a desired size range needs to be removed during classification in order to obtain a particle size distribution of desired range.
To overcome the above drawback, it has been proposed an apparatus for producing fine resin particles by drawing a toner raw material extruded from a kneader into a fibrous shape using a roller (drawing) and cutting it with a cutter (Japanese Patent Application (JP-A) No. 06-138704).
The apparatus disclosed in JP-A 06-138704 intends to obtain a resin powder with a narrow particle size distribution by kneading and heating a resin as a toner raw material in a kneader, extruding the melted resin through a die into a string shape, drawing the string-shaped resin into a fibrous shape by a roller followed by solidification, and cutting the produced fibrous resin.
However, in the apparatus disclosed in JP-A 06-138704, the resin extruded from the kneader is drawn into the fibrous shape by the roller. In case that the resin is broken in a drawing step by any cause, the fibrous resin cannot be fed to a next step of cutting step, and then production of the fine resin particles have to be interrupted or variation in diameters of the fibrous resin is caused.
This may lead to reduced production efficiency, which will be a big problem in terms of production of fine resin particles on a commercial scale. When the fine resin particles are produced by cutting the fibrous resin, variation is caused in the particle diameters. Moreover, in the method for drawing the fibrous resin by the roller, it is generally difficult to stably produce a fibrous fine resin having a diameter of 10 μm or less on a commercial basis, unless particular methods such as a method for producing a sea-island structure composite fiber using an incompatible two-component polymer blend, and a method for producing easily splittable fiber are used in combination.
Thus, in the method disclosed in JP-A 06-138704, it is practically impossible to stably and efficiently produce a fine fiber using the commonly used resin material, specifically, a resin material which is not optimized for making into the fiber.
As an apparatus for efficiently producing a fibrous fine resin, a spinning die for a melt-blow type nonwoven fabric has been proposed (Japanese Patent Application (JP-A) No. 2002-371427).
The spinning die disclosed in JP-A 2002-371427 is so configured as to extrude a melted resin along with hot air from a nozzle, and then introduce the extruded resin along with cold air to an outlet of spinning machine so as to cool and make the resin into a fibrous shape.
By the method disclosed in JP-A 2002-371427, even if the resin is broken by any cause, diameter variations are hard to occur because drawing is performed immediately after the raw material resin has been discharged from the spinning die.
The spinning die described in JP-A 2002-371427 has been originally developed for producing a nonwoven fabric and not been intended for other applications.
Therefore, it is necessary to find a new optimum operational condition of the spinning die in order to use the spinning die disclosed in 2002-371427 for producing toner and fine resin particles intended for powder coating, and there leaves room for improvement in order to combine the spinning die in an apparatus for producing fine resin particles, for example, devising arrangement of the spinning die.
Attempts have been made to apply the spinning die and apparatus disclosed in JP-A 2002-371427 to an apparatus for processing toner raw material into a fibrous shape (see JP-A 2004-332130).
Specifically, JP-A 2004-332130 discloses an operational condition and installation condition under which the apparatus of JP-A 2002-371427 is applied to a toner. That is, JP-A 2004-332130 discloses a cooling mechanism and its arrangement, and optimum conditions such as optimum temperature and amount of air used for drawing as operational conditions on the basis of the apparatus of JP-A 2002-371427.
Moreover, JP-A 2006-106235 discloses a production method and apparatus contains a requirement ([0020] and FIG. 2) similar to a requirement disclosed in JP-A 2004-332130, and a static mixer disposed after kneading step and before pulverizing part is provided, and an operational condition is mentioned as well.
Furthermore, it is proposed that a pulverizing step after the toner raw material is processed into the fibrous shape and shape control taking advantage of a post-step (JP-A 2006-106236).
However, JP-A 2004-332130 leaves room for improvement, for example, devising the spinning die suitable for the toner raw material, shape of a nozzle hole, and structures of the apparatus for feeding the toner raw material to the spinning die, and flow path.
Moreover, there is room to propose a process for raw material which is suitable for the method disclosed in JP-A 2004-332130 in terms of a surface of the raw material.
JP-A 2006-106235 leaves room for improvement in the apparatus and structure, as well.
JP-A 2006-106236 resulted from an improvement of various processes does not disclose improvement of particle size distribution, and the particle size distribution of the toner is needed to be improved by improving a pre-process of a method for processing a toner into a fibrous shape.
The concept of JP-A Nos. 06-138704, 2002-371427, 2004-332130, 2006-106235, and 2006-106236 are such that a raw material is efficiently distributed to be a uniform size in advance, and then cut or pulverized so as to have toner particles having a sharp particle size distribution as an final product, and have been broadly studied for the purpose of improvement of yield and reduction of energy. However, in JP-A Nos. 06-138704, 2002-371427, 2004-332130, 2006-106235, and 2006-106236, a toner is processed into a fibrous shape, and recovered once as it is, and then the fibrous-shaped toner is pulverized or cut to obtain fine particles by means of a secondary apparatus. Thus, it leaves much room to improve efficiency of pulverization.
JP-A 2005-004182 discloses a technique in which efficiency of pulverization is improved and generation of fine powder is suppressed by pulverizing a toner material containing a gaseous substance finely dispersed in a resin. This is an excellent idea for reduction of pulverization energy, but variations of particle size distribution occurs as in the case of conventional pulverization and classification.